Bind condition choices when admin form is created

[callorico]

Previously, the FlagStateForm condition choices were bound when the module was loaded. It is possible for this happen before custom conditions in other Django apps have been registered.

This commit delays populating the choices until the form is created. This allows all custom conditions to be registered first.

[coveralls]

Pull Request Test Coverage Report for Build 59

• 12 of 13 (92.31%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 99.869%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flags/tests/test_forms.py	8	9	88.89%
<!--	Total:	12	13	92.31%	-->

Files with Coverage Reduction	New Missed Lines	%
flags/urls.py	2	100.0%
<!--	Total:	2		-->

Totals
Change from base Build 56:	-0.1%
Covered Lines:	763
Relevant Lines:	764

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 50

• 12 of 13 (92.31%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 99.607%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flags/tests/test_forms.py	8	9	88.89%
<!--	Total:	12	13	92.31%	-->

Files with Coverage Reduction	New Missed Lines	%
flags/urls.py	2	95.65%
<!--	Total:	2		-->

Totals
Change from base Build 38:	-0.4%
Covered Lines:	761
Relevant Lines:	764

---

💛 - Coveralls

[willbarton]

@callorico Thanks! I'll make sure it gets a review, but for now #6 should fix the Travis error if you rebase or update from master.

[callorico]

Thanks. I've rebased.